Pultrusion is a manufacturing process for producing continuous lengths of fiber reinforced plastic (“FRP”) structural shapes. Raw materials used in pultrusion processes include a liquid resin mixture (containing resin, fillers and specialized additives) and reinforcing fibers. The process involves pulling these raw materials, rather than pushing as is the case in extrusion, through a heated steel forming die using a continuous pulling device. The reinforcement materials are in continuous forms such as rolls of fiberglass mat or doffs of fiberglass roving. The two ways to impregnate, or “wet out”, the glass are open bath process and resin injection. Typical commercial resins include polyester, vinyl esters, phenolics, and epoxy compounds. These resins usually have very long gel times and can be run in an open bath process in which the reinforcing fibers are soaked in a bath of resin and the excess resin is scraped off by a series of preform plates and at the die entrance. As the wetted fibers enter the die, the excess resin is squeezed through and off the reinforcing fibers. The pressure rise in the die inlet helps to enhance fiber wet-out and suppresses void formation. As the saturated reinforcements are pulled through the die, the gelation (or hardening) of the resin is initiated by heat from the die and a rigid, cured profile that corresponds to the shape of the die is formed.
For resin systems like polyurethanes which have a fast gel time and a short pot life the resin injection process is used. In the injection process, the reinforcement materials are passed through a small closed box which is usually attached to the die or may be part of the die. The resin is injected under pressure through ports in the box to impregnate the reinforcement materials. Resin injection boxes are designed to minimize resin volume and resin residence time inside the box. There are a number of different resin injection box designs in the literature all of which have the common features of an angled or tapered design and the exit profile matching the shape of the die entrance.
The patent literature provides a number of teachings with respect to polyurethane pultrusion. For example, U.S. Pat. No. 6,420,493, issued to Ryckis-Kite et al., discloses a two component chemically thermoset composite resin matrix for use in composite manufacturing processes. The matrix includes a solvent-free polyisocyanate component and a solvent-free polyol component. The solvent-free polyisocyanate component is taught to be an aromatic polyisocyanate, an aliphatic polyisocyanate or a blend of both. The disclosed solvent-free polyol component is a polyether polyol, a polyester polyol or a blend of both. The polyisocyanate component and the polyol component are said to be in relative proportions in accordance with an OH/NCO equivalent ratio of 1:1 to 1:2. Ryckis-Kite et al. require the presence of 10%-40% of a polyester polyol with the use of 5 to 20 wt. % of a hydroxyl terminated vegetable oil also being taught. For the isocyanate component, Ryckis-Kite et al. state that it is preferred to have at least 15 wt % of an aliphatic polyisocyanate.
Cheolas et al., in U.S. Pat. No. 6,793,855, teach polyisocyanurate systems, pultrusion of those systems to produce reinforced polyisocyanurate matrix composites, and the composites produced by that pultrusion process. The polyisocyanurate systems of Cheolas et al. include a polyol component, an optional chain extender, and an isocyanate. The polyisocyanurate systems are said to have extended initiation times of about 5 minutes to about 30 minutes at room temperature and to be capable of snap curing. Cheolas et al., at col. 8, lines 10-23, state that in their Type I polyisocyanurate systems, the polyol, chain extender and isocyanate may be varied to control the miscibility of the reaction mixture. Several methods designed to increase miscibility of the reaction mixture are disclosed. Cheolas et al. teaches that substantial polymerization of the polyurethane takes place in the impregnation die.
U.S. Pat. No. 7,056,976, issued in the name of Joshi et al., also discloses polyisocyanate-based reaction systems, a pultrusion process using those systems to produce reinforced matrix composites, and composites produced by that pultrusion process. The polyisocyanate-based systems are mixed activated reaction systems that include a polyol composition, an optional chain extender or crosslinker and a polyisocyanate. The polyisocyanate-based systems are said to exhibit improved processing characteristics in the manufacture of fiber reinforced thermoset composites via reactive pultrusion. Joshi et al. teach that gel times are the key parameter in polyurethane pultrusion.
In addition, Cheolas et al., in U.S. Published Patent Application No. 2004/0094859 A1, teach polyisocyanurate systems, pultrusion of those systems to produce reinforced polyisocyanurate matrix composites, and composites produced by that pultrusion process. The polyisocyanurate systems disclosed in this published application include a polyol component, an optional chain extender and an isocyanate. The polyisocyanurate systems are said to have extended initiation times of from about 5 minutes to about 30 minutes at room temperature, and are capable of being snap cured. Cheolas et al., like Joshi et al., teach that gel times are the key parameter in polyurethane pultrusion processes.
Polyurethane-forming systems currently being used to produce composites by pultrusion are generally based upon aromatic polyisocyanates. The physical properties of these systems are very good when used in applications where the composite is used indoors and is not therefore subject to weathering. However, these aromatic polyisocyanate-based systems do not perform well in unprotected outdoor applications. Poor weathering characteristics of the resin will result in color change, gloss loss and deterioration of the resin matrix and exposure of bare glass fibers (commonly referred to as “blooming”).
Aliphatic isocyanate-based polyurethane-forming systems are known to have better weathering characteristics than the aromatic isocyanate-based systems. However, substitution of an aliphatic isocyanate for the aromatic isocyanate in known pultrusion systems produces composites with better weathering characteristics but inferior physical properties.
A need therefore exists for improved polyurethane formulations for use in pultrusion processes to produce composites with both excellent physical properties and excellent weathering properties.